Potassium chloride is an essential plant nutrient and is mainly used as fertilizer either as single fertilizer or in combination with other plant nutrients. Potassium chloride is also used in several industries like dyes, soaps, detergents, food, pharmaceuticals and also as starting material for preparing other potassium chemicals like potassium hydroxide and potassium carbonate. The purity requirement for these applications is more stringent, requiring purity >98.0% in most cases, Low sodium salt, which is a mixture of sodium chloride and potassium chloride, is a balanced salt especially suitable for persons suffering from hypertension and heart diseases.
J. H. Hildebrand (‘Extraction of Potash and other Constituents from sea water Bittern’, Journal of Industrial and Engineering Chemistry, vol. 10, no. 2, 1918 pp 96–106) discloses theoretical aspects of recovery of potash from sea bittern and proposes a process for extraction. According to this process, bittern is evaporated at a temperature between 100–120° C., thereby forming a solid mixture of sodium chloride and kieserite (MgSO4.H2O), separating this mixture under hot conditions in a heated centrifuge, and cooling the mother liquor in a cooler for separation of carnallite. Carnallite is decomposed and washed with water to produce potassium chloride. The drawback of this process is that it is demanding in terms of energy requirement and KCl obtained from the carnallite is not of industrial grade.
K. Seshadri et al (‘Manufacture of Potassium chloride and byproducts from Sea Bittern’ Salt Research and Industry, April–July 1970, Vol. 7, page 39–44) discloses further concentration of bittern in solar pans. After removing crude salt and Sels' mixts [mixture of NaCl and Epsom salt (MgSO4.7H2O)], mixed salt [mixture of NaCl, MgSO4.7H2O, KCl.MgSO4.3H2O (kainite) and MgCl2.6H2O is formed in solar pans. Mixed salt is dispersed with high density bittern in proper proportion and heated to a temperature of 110° C. when kieserite (MgSO4.H2O) is formed which is separated by filtering the slurry under hot conditions. The filtrate is cooled to ambient temperature, when carnallite (KCl.MgCl2.6H2O) crystallizes out. Carnallite is decomposed with water to get a solid mixture of sodium chloride and potassium chloride while magnesium chloride goes into solution. Solid mixture of potassium chloride and sodium chloride is purified using known techniques to produce pure potassium chloride. The drawbacks of this process are: Mixed Salt (containing kainite) is obtained only after two earlier solid evaporates, i.e., crude salt and sels' mixt, are removed separately. This is done by solar evaporation in pans, removal of salts from pans, and pumping of liquid into intermediate pans—all of which are highly labour intensive. Kainite type mixed salt is to be processed further by mixing the same with high-density bittern and using hot extraction technique followed by cooling to exact carnallite from mixed salt. This is a tedious and energy intensive operation and the efficiency of KCl recovery is low. Carnallite which is obtained by this process contains substantial quantity of sodium chloride which remains along with potassium chloride after decomposition of the double salt, and requires to be purified by the process of hot leaching which is energy intensive.
In U.S. Pat. No. 3,099,528, dated Jul. 30, 1963, entitled “Recovery of values from Natural Lake and Sea Brines” carnallite is claimed to be produced as one of the products. However this carnallite contains substantial quantity of sodium chloride and on decomposition produces sylvinite which again requires floatation to produce potassium chloride.
In another process being followed by Kali-und Salz in Germany, KCl is produced from sylvinite through an electostatic separation technique. This process is energy intensive.
In Ullmann's Encyclopedia of Industrial Chemistry, Sixth Edition, 1999, under the Chapter, Potassium compounds, carnallite is produced in normal manner by solar evaporation of bittern obtained from Dead Sea. However the carnallite contains large quantity of sodium chloride impurity with the result that the product obtained after decomposition with water is potassium chloride contaminated with substantial quantity of sodium chloride which requires froth floatation or hot leaching operations to produce potassium chloride of fertiliser grade quality. Hot leaching process involves processing carnallite decomposed product (CDP) with equilibrium liquor (saturated solution of sodium chloride and potassium chloride) at high temperature, filtering hot to remove solid sodium chloride and cooling the filtrate to produce potassium chloride. The main problem with this technique is that during the process of cooling of filtrate for crystallization of KCl, some sodium chloride also comes out, with the result potassium chloride is contaminated with about 3% NaCl. Secondly, on account of CDP containing about 0.5% to 1.0% Mg+2, the same is extracted into the equilibrium liquor, which reduces the extraction efficiency of the liquor. In case of froth floatation, organic frothing agents have to be added for purification of potassium chloride, which may be hazardous to soils and the environment in general. In another reported process, called cold crystallisation technique, sodium chloride is removed from carnallite by fractional crystallisation including wet sieving thereby producing carnallite containing about 4 to 5% NaCl.
Chinese Patent CN 1275531 dated Dec. 6, 2000 by Song Youlin (CN) titled “Method for directly obtaining low sodium carnallite” describes addition of solid bischofite into bittern to crystallise out low sodium carnallite from which potassium chloride can be produced. In this process it may be necessary to evaporate the bittern obtained after recovery of carnallite to very high densities to obtain bischofite. No mention is made of the efficiency of recovery and how the residual K+ in the effluent is dealt with Chinese Patent CN 1127219, dated Jul. 24, 1996, by Song Yioian (CN) and Jinyu Zhou (CN), titled “Preparing large crystal high purity potassium chloride by adding halogen method to remove Na and isolate K” discloses blending of solid carnallite with different types of bittern.
Chinese Patent CN 1248549 dated Mar. 29, 2000 by Li Lianglin (CN), Li Yunping (CN) and Wei Xinjun (CN), titled “Process for preparing high-quality potassium containing products by salt-field method” discloses extraction of potassium, wherein bittern is first introduced in sodium pool, during day time when temperature is high, thereby separating out salt minerals of sodium chloride etc., and during night time when temperature is low, the bittern is introduced into potassium pool to separate out potassium salts. The process is repeated several times till potassium ion content of bittern is less than 2/1000. However, as mentioned in the patent, potassium content of product is about 52%. Moreover, such operations even though interesting in concept have practical limitations.
U.S. Pat. No. 3,994,531, dated Nov. 30, 1976, by Dillard Jr.; David S; Davis II; J. Gilbert; Every; and Richard L., entitled “Method of solution mining potassium chloride from subterranean deposits”; discloses an improved method of solution ruining of potassium chloride from subterranean ore deposits containing both potassium chloride and sodium chloride. In this process an aqueous medium is passed through ore deposits so that potassium chloride and sodium chloride are dissolved therein. In a second subterranean deposit containing magnesium chloride, aqueous medium is also passed so as to obtain a solution of magnesium chloride. Both these solutions are combined and an aqueous solution containing potassium chloride, sodium chloride and magnesium chloride in specific proportion is produced, and solubility of potassium chloride is reduced. The combined aqueous brine solution is concentrated and concentrated solution is cooled to cause the precipitation of substantially pure potassium chloride. Application of the process to sea water systems may, however, not be straightforward.
U.S. Pat. No. 4,140,747 dated Feb. 20, 1979, by Sadan and Abraham, titled “Process for production of potassium chloride and magnesium chloride” discloses a process wherein potassium chloride and magnesium chloride hexahydrate (Bischofite) are produced from carnallite or carnallite containing sodium chloride. The process comprises heating carnallite at a temperature of 70° C. in the presence of added water or heating to 167.5° C. without added water. Solid potassium chloride is then separated. In the residual brine, carnallite is separated by evaporation or by lowering temperature and it is recycled to starting stage. Residual solution consists essentially of magnesium chloride, which is recovered as bischofite. Heating of carnallite is carried out under pressure and lowering of temperature is done by flash evaporation. This process makes no special reference regarding the purity of KCl obtained.
In all of the above processes, which have focus on a single product, only a part of the KCl in bittern is actually recovered and the rest has to be recycled to increase yield of KCl. Moreover, even in those cases where the purity of KCl is high, it is well known that production of KCl in the field through carnallite can lead to deposition of insoluble particles, including organic matter, in the carnallite which are difficult to separate. These insolubles affect the quality perception of the product.
U.S. Pat. No. 3,099,528 dated Jan. 10, 1962 titled “Recovery of values from Natural Lake and Sea Brines” discloses a process wherein the main objective is to produce KCl via sylvinite obtained from crude carnallite. Calcium Chloride required to desulphate the brine in order to promote carnallite formation is produced by passing heavy gypsum slurry to an ion-exchange system utilizing cationic exchange resin in its hydrogen form. In the ion-exchange system, Calcium ions of the influent gypsum slurry are exchanged for hydrogen ions of ion-exchange resin, yielding H2SO4 as the effluent. Upon eluting the Calcium loaded resin with HCl eluent, an eluate solution of Calcium Chloride is obtained. This exchange must be carried out at elevated temperature.
UK Patent 1500288 dated Aug. 2, 1978 titled “Purification of Brine”, discloses a process wherein the sulphate content of an alkali chloride containing brine is reduced to as low as 2 g/L of sulphate by adding a stochiometric excess of calcium chloride while maintaining the pH of brine in the range 7 to 9 by the addition of HCl in the temperature range 30° C. to 50° C. The application discussed is direct utilization of brine for production of chloralkali through a purification process. While use of outsourced CaCl2 would be alright for desulphatation of small quantities of sulphate, it would be advantageous to generate CaCl2 from bittern and time, taking further advantage of co-generation of Mg(OH)2 as a useful by-product.
Chr. Balarew, D. Rabadjieva and S. Tepavitcharova (‘Improved Treatment of Waste Brines’ International Symposium on Salt 2000, page 551–554) disclose recovery of marine chemicals. In the process described, bittern is desulphated with the objective of recovering gypsum and magnesium hydroxide in pure form prior to recovery of KCl. The CaCl2 required for desulphatation of bittern is generated from the reaction of lime and desulphated bittern. The principal drawbacks of this process are; (i) by separating out the Mg in bittern prior to recovery of KCl, no advantage is taken of the lower solubility of carnallite vis-à-vis KCl which would have enabled KCl to be obtained with less of evaporation of bittern, (ii) the KCl would inevitably be contaminated with large quantites of NaCl which would make the process of purification both tedious and energy intensive, and (iii) since desulphated virgin bittern contains significant quantities of boron, the MgO that would be produced from Mg(OH)2 by the method described would inevitably be contaminated with high impurity levels of B2O3.
U.S. Patent Application 20030080066 dated Oct. 29, 2001 by Vohra, Rajinder N. et al. discloses an integrated process for recovery of high purity salt, potassium chloride, and end bittern containing 7.5 g/L Br. The process is based on desulphatation of brine with Distiller waste of soda ash industry or calcium chloride generated from limestone and acid. The main drawback of the patent application is that the process is less attractive when Distiller waste is not available in the vicinity and the carnallite obtained is contaminated with high extent of NaCl necessitating further purification.
Reference is made to “The Heinz Handbook of Nutrition” by Benjamin and Burton, published by McGraw Hill Book Co. Second Edition, page 132–133, wherein it is mentioned that the dietary needs for potassium roughly equals that of sodium.
Alves de Lima et al. in Brazilian Patent BR 9806380 A, 12 Sep. 2000, titled “Production of dietetic salt by mixing”, state that low sodium dietetic salt is produced by mixing sea salt with potassium chloride, potassium iodate and sodium aluminum silicate, thereby mixing 4 parts of sodium chloride with 6 parts of potassium chloride. The drawback of this process is that one has to separately procure sodium chloride and potassium chloride and blend them together so as to make a solid mixture. Apart from the fact that components of low sodium salt are first made in pure form and then remixed which would clearly be costlier than if such a mixture could be obtained directly, there may also be difficulty in preparing a truly solid homogeneous mixture.
Chinese Patent CN 1271541 A, 1 Nov. 2000 to Shuqing Wang in, titled “Multi-element low sodium nutritive sale”, discloses preparation of low sodium nutritive salt by crystallising salt from saturated brine under vacuum. The salt is then mixed uniformly with salts such as KCl and MgSO4.7H2O, followed by mixing with KIO3 and Na2SeO3 solutions, drying and finally mixing with active Ca and Zn lactate. The drawback of this process is that apart from the difficulty of mixing various constituents in a homogeneous solid mixture, salt is to be crystallized from hot saturated brine involving high energy consumption thereby increasing the cost of production.
Japanese Patent Abstract No. 02022122 dated Jan. 25, 1990, titled “Production of Composite Salt consisting of Sodium Chloride and Potassium Chloride” discloses a process wherein sea water is selectively concentrated through an Ion-exchange membrane and further concentrated in a vacuum evaporator to get desired level of NaCl and KCl in solution. The process requires sophisticated ion-exchange membrane technology and would be highly energy intensive as large volumes of water will need to be evaporated.
PCT International Application No. PCT/IN02/00018 dated Jan. 31, 2002 discloses a method of producing KCl-enriched low sodium salt from bittern. One drawback of the method is that, in case the crude carnallite is produced in the field, it can be contaminated with insoluble matter such as dust, trapped organic matter, ubiquitous black particles, etc., which are difficult to remove from the carnallite decomposed product unless recourse is taken to re-dissolution, filtration and recrystallisation which, therefore, defeats the purpose of the invention.
Reference is made to the Chapter in Ullmann's Encyclopedia of Industrial Chemistry, Sixth Edition, 2002 (Electronic Version) dealing with Magnesium Compounds written by Margarete Seeger, Walter Otto, Wilhelm Flich, Friedrich Bickelhaupt and Otto, S. Akkerman, wherein the process of preparation of magnesium hydroxide from seawater is described. It is mentioned therein that preparation of low boron containing magnesia requires over liming of the seawater up to pH 12 to maintain B2O3 content less than 0.05% in magnesia. Overliming involves higher lime cost, need for neutralization of supernatant and results in a colloidal suspension which is more difficult to filter. Another drawback not circumvented is the formation of calcium chloride-containing effluent, which is discharged back into the sea.